The invention pertains generally to high fidelity turntable systems and is more particularly directed to a low aggregate resonance substructure for mounting such systems.
Turntable designers in the past have exerted much concentration on reducing acoustic distortions in high fidelity systems. Their efforts have primarily been directed to decreasing such distortions as wow and flutter, which are speed inconsistencies, or rumble, which is the basic noise level inherent in the rotating motion of a system that is received by a phonographic cartridge during play. So much success has been achieved in alleviating these two distortions that they have virtually been eliminated as problems to the serious audiophile.
One acoustic distortion problem for which much less progress has been made is resonance. This serious distortion can be described and summarized as the sounds produced by the substances of which the turntable system is constructed. Further, depending upon the acoustic and mechanical coupling of the turntable to its environment, these substances may also include the objects on which the turntable is mounted or even resting. The resonant sounds produced by these substances are created by and amplified along with the musical or other information that the phonographic cartridge is processing, thereby producing distortion. The problem usually manifests itself as a "thump" or a loss of fidelity in the lower, or even middle, frequencies and is sometimes referred to as "one note bass".
Generally, the cause of the problem is accepted as being one of feedback, as the turntable substances will not make sounds by themselves. Thus, it is the excitation of these substances by the sound system itself which produces the resonance and consequent distortion.
It is believed that there are three principal feedback paths in a turntable system. The first is through the air from the acoustic energy produced by the speakers to the turntable. Energy in the form of sound vibrations excite the turntable system construction through this acoustic path to produce a resonant distortion.
The second principal path is a mechanical one whereby the mechanical vibrations of the speakers and the vibrations caused by their acoustical energy in other surrounding objects are coupled back through any solid object in contact with the mounting structure of the turntable. The closer the speaker is to the turntable system and the more direct the mechanical coupling, the more distortion this feedback path creates.
The third path which has not been previously compensated or described is an inherent feedback path where the actual sound vibrations produced by the cartridge transducing the stylus motion into electrical signals provide an excitation for the resonant substances comprising the turntable.
Generally, the prior art has attempted to solve these resonance problems by providing a very high mass structure for mounting the turntable assembly and by decoupling this high mass structure from the environment with resilient members such as springs. The high mass structure will require more acoustic energy to resonate and, thus, will alleviate the acoustic feedback while the decoupling springs will isolate the high mass structure from mechanical vibration and substantially limit the mechanical feedback.
The high mass of the turntable supporting structure, however, may exacerbate the inherent feedback as there is a greater amount of physical material present to be heard by the tone arm assembly. Further, a massive mounting structure is bulky to handle and necessitates a stronger and more expensive set of decoupling springs.
In addition, many modern amplification systems have equalization factors where lower notes are amplified more than higher notes. This is to compensate the method used for the mechanical recording medium where lower notes produce less physical movement of the stylus and, thus, less signal. Lower resonant tones will, consequently, be amplified, along with low note information, more than the higher resonant tones. It would, therefore, be highly desirable to eliminate as much of the low resonant feedback from a turntable system as possible, even to the extent of increasing the high resonant tones.